Wet type clutches usually include two or more mating plates which rotate in conjunction with one shaft and friction plates disposed alternately between the mating plates so as to rotate in conjunction with another shaft. As a piston operates, those plates come into friction engagement with each other, and a torque may be transmitted between those two shafts. Mating plates and friction plates are soaked in lubricating oil so as to be cooled down by lubricating oil. Friction material is bonded on the faces of the friction plates opposing the mating plates (both sides). Oil grooves are usually provided between the adjacent friction material segments, or in the friction materials in the radial direction so that lubricating oil may be discharged by a centrifugal force to the perimeter sides of the friction plates.
When the piston is not operating in a wet type clutch, namely the clutch is out of engagement, it is ideal that a torque not be transmitted at all between the mating plates and friction plates so as to not cause power loss. In practice, however, in a wet type clutch even in non-engaging-of-clutch state, a small amount of torque is likely to be transmitted between the mating plates and the friction plates. This is an occurrence of drag torque leading to a power loss through a lubricant at the time of disengagement of the clutch based on the shear resistance force of the lubricant.
It is known that the drag torque may be reduced by increasing the content of air in the lubricating oil. Also, in order to reduce the drag torque, friction plates provided with various-shaped friction materials and oil grooves have been devised so far, and some are disclosed in the following examples.